1. Field of the Invention
Embodiments of the present invention generally relate to semiconductor processing and, more particularly, to forming a conductive contact for interfacing with a semiconductor device.
2. Description of the Related Art
One or more Layers of Gallium Nitride (GaN) are often deposited when fabricating various types of semiconductor devices. For example, during the fabrication of light emitting diodes (LEDs), an epitaxial structure of an “LED stack” including layers of p-doped GaN (p-GaN) and n-doped GaN (n-GaN) may be formed. FIG. 1 illustrates an example of such a structure 104, having an n-GaN layer 106 and a p-GaN layer 110 separated by a multi-quantum well MQW layer 108. The structure 104 is typically deposited on a substrate 102 of suitable material, such as c-plane SiC, c-plane sapphire.
The deposition may involve gaseous reactions, such as 3GaCl+3NH3→3GaN+2H2+3HCl or TMG+NH3→GaN+CHx+H2. Such deposition on c-plane SiC, c-plane sapphire typically results in growth of crystalline structure along the c-axis of hexagonal crystal configuration with a surface where atomic bonds between Ga and N has a specific configuration as shown in FIG. 2A. This surface is usually very stable as the atoms normally occupy the lowest free energy states during the deposition process.
As illustrated in FIG. 3, contacts 118 are typically formed in order to apply a voltage differential across the n-GaN and p-GaN layers to cause the structure 104 to emit light. In conventional LED fabrication processes, the n-pad contact to the n-GaN layer 106 is formed as a metal contact on top of as-deposited n-GaN layer 106. Conventional metal contact structures using Ni, Cr, Ti, and Pt have been used as contact materials for as-deposited surfaces. Specific materials, such as Cr/Au, Ti/Al/Ni/Au, and Ti/Ni/Au have yielded relatively low resistance contact to the n-GaN. Ni/Au and ITO have been used for making ohmic contact to the p-GaN.
In some cases, it may be beneficial to form semiconductor structures using processes that result in crystal structures with inverted configuration of atomic bonds along the c-axis of the crystal structure (in the direction towards the surface) between Ga and N atoms. The inverted configuration of atomic bonds is realized by removing the original substrate (c-plane sapphire, c-plane SiC) and exposed the inverted surface; this inverted surface is shown in FIG. 2B. For example, such structures may be formed when fabricating a vertical LED structure, as described in U.S. patent application Ser. No. 11/032,882, filed Jan. 11, 2005 and herein incorporated by reference in its entirety. As described therein, Vertical LED devices (VLEDs) may have many advantages, for example, better thermal dissipation, better current distribution, higher efficiency.
As illustrated in FIG. 4A, in the VLED fabrication process, a reflective surface 200 and metal substrate 202 may be deposited or bonded on top of as deposited Gallium terminated p doped-GaN surface. The carrier substrate 102 (for example c-plane sapphire, c-plane SiC,) may be removed from the structure leaving behind and exposing the GaN that was in contact with the carrier substrate as shown in FIG. 4B. Referring ahead to FIG. 4C, the VLED structured with the carrier substrate 102 removed has the exposed layer 106 GaN facing up. The corresponding crystal structure is shown in FIG. 2B, with the atomic bond between Nitrogen atoms 119 and the Gallium atoms 117 in inverted configuration compared to that shown in FIG. 2A.
As a result, this new created GaN surface/inverted surface may exhibit very different properties with respect to the as-deposited GaN surface shown in FIG. 2A due to a mirrored configuration of Nitrogen and Ga atoms. In the present application, this newly created n-doped GaN layer having a surface exposed by removing a substrate on which the GaN layer was formed, will be referred to herein as an Nu-GaN layer. This exposed surface could be treated by mechanical or chemical means, for examples: wet etching, dry etching, polishing, and/or lapping.
The spontaneous polarization at the Nu n-doped GaN surface has an opposite direction compared to that of conventional as deposited GaN surface. The Nu-GaN is a man-made surface (e.g., resulting from removing a deposition substrate) and the crystal structure of such a Nu-GaN surface may be less stable than as deposited GaN surface, wherein stability refers to the uniformity of contact resistance when forming contacts. As a result, forming low resistance electrical contacts on Nu-GaN is not well known. In conventional LED technology, the metallization techniques for as-deposited n-doped or p-doped GaN layers are well understood, but metallization techniques for Nu-GaN is not well understood. For VLED, the contact to p electrode is contacted to conventional p-doped GaN layer surface, but the n-electrode may have to make contact to Nu-n-doped GaN layer and it is important to have a low resistance and stable contact for Nu-n-doped GaN for reliable VLED devices.
Accordingly, what is needed is technique for forming a contact for interfacing with a Nu n-doped GaN semiconductor layer that provides low and stable resistance.